It is known to generate a dynamic focus voltage that is applied to a focus electrode of a cathode-ray tube (CRT) for changing the focus of the CRT as the length of the beam path changes during horizontal and vertical scanning. For example, U.S. Pat. No. 6,300,731, entitled DYNAMIC FOCUS VOLTAGE AMPLITUDE CONTROLLER, in the name of John Barrett George, describes such an arrangement. There, a parabolic horizontal rate voltage is applied to a focus voltage amplifier.
A data sheet of a deflection processor integrated circuit (IC) TA1317AN published by Toshiba Corporation describes generation of a parabola waveform for horizontal dynamic focus. The peak-to-peak amplitude of the parabola waveform is selectable such that the level of the parabola at the center of horizontal trace remains the same at the different selections of the peak-to-peak amplitude. Whereas, at the end of trace, the peak level of the parabola is different at the different selections of the peak-to-peak amplitude.
Also, between the end of one trace and the beginning of the next trace, a parasitic or undesirable waveform portion is generated in IC TA1317AN. When applied to the focus voltage amplifier, this parasitic waveform portion, that occurs, during retrace, may produce an undesirable distortion in a portion of the dynamic focus voltage that occurs outside retrace, in a visible portion of the horizontal scan line, near the end of trace. It may be desirable to eliminate the parasitic waveform portion without disturbing the portion of the dynamic focus voltage that occurs in the vicinity of the end of trace. It may also be desirable to do so in a manner that preserves the ability to adjust the peak-to-peak amplitude of the parabola waveform.
In carrying out an aspect of the invention, a first parabola waveform is capacitively coupled via a capacitor to a first semiconductor switch responsive to a first reference voltage and forming a clamp. The clamp generates a second parabola waveform having a peak level, at the end of trace, that is determined by the first reference voltage. The clamping operation of the first semiconductor switch causes the capacitor to charge in a manner to provide level shifting. Advantageously, at selectively different peak-to-peak amplitudes of the first parabola waveform, the peak amplitude of the second parabola waveform remains the same. Thus, the level shifting, advantageously, preserves the ability to select different peak-to-peak amplitudes of the second parabola waveform. A second semiconductor switch, responsive to a periodic switch control signal and to a second reference voltage, replaces in the second parabola waveform an undesirable waveform portion of the first parabola waveform that occurs between the end of trace and the beginning of the immediately following trace of the second parabola waveform with a constant level. The constant level is determined in accordance with the second reference voltage. Thereby, the undesirable waveform portion is, advantageously, removed.